Vinyl aromatic-1,3-diene copolymers stabilized with protection colloids used for modifying materials containing plaster or calcium carbonate

ABSTRACT

The vinyl aromatic-1,3-diene copolymers stabilized with protection colloids are used in the form of aqueous polymer dispersions or water-redispersible polymer powders for modifying materials that contain plaster or calcium carbonate. The polymer dispersions or polymer powders are obtained by the emulsion-polymerization of a mixture containing at least one vinyl aromatic and at least one 1,3-diene in the presence of at least one protection colloid and without any emulsifier, and by optionally drying the aqueous polymer dispersions thus obtained.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The invention relates to the use of protective colloid-stabilizedvinylaromatic-1,3-diene copolymers in the form of their aqueous polymerdispersions or polymer powders which are redispersible in water for themodification of gypsum-based materials or of materials based on calciumcarbonate.

2) Background Art

Gypsum is a building material which is available in large quantities atlow cost. Apart from naturally occurring forms of gypsum, considerableamounts of gypsum are obtained from flue gas desulfurization plants andthere is worldwide interest in utilizing these. Apart from the use ofgypsum in jointing compositions, gypsum mortars are used, in particular,for interior plasters and coatings. Since gypsum cures with an increasein volume, it is the ideal binder for coatings and moldings in order toavoid possible crack formation. However, owing to theirwater-sensitivity which is reflected, inter alia, in an unsatisfactoryfreezing/thawing behavior, renders comprising plaster of Paris asbinder, with or without proportions of builder's lime, may, according toDIN standard 18550, be used only for interior plasters subject tostresses customary for these, but not for damp rooms or for externalrenders. In order to be able to use gypsum-based building materials forexternal applications and wet applications, too, they have to besufficiently hydrophobicized.

In DE-A 3704439 (U.S. Pat. No. 4,851,047), the use of silicones andsiloxanes, steareates and paraffin waxes is proposed forhydrophobicizing gypsum mortars. EP-A 320982 describes the use ofredispersion powders based on vinyl acetate-Versatic acid-vinyl estercopolymers for hydrophobicizing gypsum-based materials. EP-A 477900discloses the use of dispersible powder compositions based on vinylester polymers or styrene-acrylate polymers as additives for improvingmechanical properties such as adhesion, abrasion resistance and flexuralstrength in gypsum, building adhesives and mortars. EP-A 728715recommends the use of compositions of dispersible powders andthixotropic additives for hydrophobicizing gypsum, with the dispersiblepowders recommended being ones based on vinyl acetate copolymers and onstyrene-acrylate copolymers.

JP-A 5/836 (Derwent Abstract AN 93-49422) describes thehydrophobicization of cement and gypsum by means of a pulverulentmixture of polysiloxane and a copolymer of vinylaromatics, dienes and/oracrylates. JP-A 57/205352 (Derwent Abstract AN 83-10505K) describes theproduction of water-resistant gypsum moldings by addition of polymerlatices of acrylate, styrene, vinyl ester and epoxy resins to the gypsummortar and subsequent heat treatment to cure the moldings.

A disadvantage of the previously mentioned hydrophobicizing agents istheir not negligible hydrophilicity, for example in the case of vinylester or acrylic ester polymers, which leads to a low water resistance.In the abovementioned Japanese publications, the emulsifier content ofthe latices employed leads to increased water absorption. This can beimproved by the addition of additives such as siloxanes, but only withincertain limits. In many cases, the mechanical strength obtained in thehydrophobicization is also unsatisfactory.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a composition forthe hydrophobicization of gypsum-based materials and of materials basedon calcium carbonate, which leads to hydrophobicized materials having ahigh water resistance and high mechanical strength without furtheradditives.

The invention provides for the use of protective colloid-stabilizedvinylaromatic-1,3-diene copolymers in the form of their aqueous polymerdispersions or polymer powders which are redispersible in water for themodification of gypsum-based materials or of materials based on calciumcarbonate, wherein the polymer dispersions or polymer powders areobtained by emulsion polymerization of a mixture comprising at least onevinylaromatic and at least one 1,3-diene, in the presence of one or moreprotective colloids and in the absence of emulsifiers, and, if desired,drying of the aqueous polymer dispersions obtained in this way.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Suitable vinylaromatics are styrene and methylstyrene; preference isgiven to copolymerizing styrene. Examples of 1,3-dienes are1,3-butadiene and isoprene; preference is given to 1,3-butadiene. Ingeneral, the copolymers comprise from 20 to 80% by weight, preferablyfrom 30 to 70% by weight, of vinylaromatic and from 20 to 80% by weight,preferably from 30 to 70% by weight, of 1,3-diene. Further monomers mayalso be present if desired, and the percentages quoted in each case addup to 100% by weight.

If desired, up to 30% by weight, based on the total weight of themonomer phase, of further monomers which can be copolymerized withvinylaromatics and 1,3-dienes, e.g. ethylene, vinyl chloride,(meth)acrylic esters of alcohols having from 1 to 15 carbon atoms orvinyl esters of unbranched or branched carboxylic acids having from 1 to15 carbon atoms, may be additionally present in copolymerized form.

If desired, from 0.05 to 10% by weight, based on the total weight of themonomer mixture, of auxiliary monomers may be additionally present incopolymerized form. Examples of auxiliary monomers are ethylenicallyunsaturated monocarboxylic and dicarboxylic acids, preferably acrylicacid, methacrylic acid, fumaric acid and maleic acid; ethylenicallyunsaturated carboxamides and nitriles, preferably acrylamide andacrylonitrile; monoesters and diesters of fumaric acid and maleic acid,e.g. the diethyl and diisopropyl esters, and also maleic anhydride,ethylenically unsaturated sulfonic acids or their salts, preferablyvinylsulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid. Furtherexamples are precrosslinking comonomers such as multiply ethylenicallyunsaturated comonomers, for example divinyl adipate, diallyl maleate,allyl methacrylate or triallyl cyanurate, or after-crosslinkingcomonomers, for example acrylamidoglycolic acid (AGA), methylmethylacrylamidoglycolate (MAGME), N-methylolacrylamide (NMA),N-methylolmethacrylamide, N-methylol(allyl carbamate), alkyl ethers suchas the isobutoxy ethers or esters of N-methylolacrylamide, ofN-methylolmethacrylamide and of N-methylol(allyl carbamate). Alsosuitable are epoxy-functional comonomers such as glycidyl methacrylateand glycidyl acrylate. Further examples are silicon-functionalcomonomers such as acryloxypropyltri(alkoxy)silanes andmethacryloxypropyltri(alkoxy)silanes, vinyltrialkoxysilanes andvinylmethyldialkoxysilanes, where alkoxy groups present can be, forexample, ethoxy and ethoxypropylene glycol ether radicals. Mention mayalso be made of monomers containing hydroxy or CO groups, for examplehydroxyalkyl methacrylates and hydroxyalkyl acrylates, e.g.hydroxyethyl, hydroxypropyl or hydroxybutyl acrylate or methacrylate,and also compounds such as diacetoneacrylamide and acetylacetoxyethylacrylate or methacrylate.

The choice of monomers or the choice of the proportions by weight of thecomonomers is made so that, in general, a glass transition temperatureT_(g) of from −70° C. to +100° C., preferably from −50° C. to +50° C.,particularly preferably from −20° C. to +40° C., results. Preference isgiven to copolymerizing styrene and 1,3-butadiene in the ratiosmentioned without further comonomers. The glass transition temperatureT_(g) of the polymers can be determined in a known manner by means ofdifferential scanning calorimetry (DSC). The T_(g) can also becalculated approximately beforehand by means of the Fox equation.According to Fox T. G., Bull. Am. Physics Soc. 1, 3, page 123 (1956):1/T_(g)=x₁/T_(g1)+x₂/T_(g2)+ . . . +x_(n)/T_(gn), where x_(n) is themass fraction (% by weight/100) of the monomer n, and T_(gn) is theglass transition temperature in degrees Kelvin of the homopolymer of themonomer n. T_(g) values for homopolymers are listed in Polymer Handbook2nd Edition, J. Wiley & Sons, New York (1975).

Suitable protective colloids are, for example, polyvinyl alcohols,polysaccharides in water-soluble form, e.g. starches (amylose andamylopectin), modified starches such as starch ethers, for examplehydroxy-alkyl ether starches, dextrins and cyclodextrins, celluloses andtheir carboxymethyl, methyl, hydroxyethyl or hydroxypropyl derivatives,poly(meth)acrylic acid, poly(meth)acrylamide, melamine formaldehydesulfonates and naphthalene formaldehyde sulfonates.

Preference is given to polyvinyl alcohols having a degree of hydrolysisof from 80 to 95 mol % and a Höppler viscosity in 4% strength aqueoussolution of from 1 to 30 mPas (Höppler method at 20° C., DIN 53015).Also suitable are hydrophobically modified polyvinyl alcohols having adegree of hydrolysis of from 80 to 95 mol % and a Höppler viscosity in4% strength aqueous solution of from 1 to 30 mpas. Examples of suchpolyvinyl alcohols are partially saponified copolymers of vinyl acetatewith hydrophobic comonomers such as isopropenyl acetate, vinyl pivalate,vinyl ethyl-hexanoate, vinyl esters of saturated alpha-branchedmonocarboxylic acids having 5 or from 9 to 11 carbon atoms, dialkylmaleates and dialkyl fumarates, e.g. diisopropyl maleate and diisopropylfumarate, vinyl chloride, vinyl alkyl ethers such as vinyl butyl ether,olefins such as ethene and decene. The proportion of hydrophobic unitsis preferably from 0.1 to 10% by weight, based on the total weight ofthe partially saponified polyvinyl acetate, with the hydrophobicallymodified polyvinyl alcohols producing a surface tension of <40 mN/m in2% strength aqueous solution. It is also possible to use mixtures of thepolyvinyl alcohols mentioned.

Particular preference is given to the partially saponified polyvinylacetates comprising vinyl alcohol units and units of vinyl esters ofalpha-branched carboxylic acids having 5 or from 9 to 11 carbon atoms inthe specified amounts. Examples of such vinyl esters are those sold asVersatic acid vinyl esters by Shell under the trade names VeoVa^(R)5,VeoVa^(R)9, VeoVa^(R)10 and VeoVa^(R)11. Further suitable polyvinylalcohols are partially saponified, hydrophobicized polyvinyl acetateswhich are obtained by polymer-analogous reaction, for exampleacetalization of the vinyl alcohol units by means of C₁-C₄-aldehydessuch as butyraldehyde. The proportion of hydrophobic units is preferablyfrom 0.1 to 10% by weight, based on the total weight of the partiallysaponified polyvinyl acetate. The degree of hydrolysis is from 80 to 95mol %, preferably from 85 to 94 mol %, and the Höppler viscosity (DIN53015, Höppler method, 4% strength aqueous solution) is from 1 to 30mPas, preferably from 2 to 25 mPas.

The greatest preference is given to polyvinyl alcohols having a degreeof hydrolysis of from 85 to 94 mol % and a Höppler viscosity in 4%strength aqueous solution of from 2 to 25 mpas (Höppler method at 20°C., DIN 53015) and their combinations with the abovementionedhydrophobically modified polyvinyl esters in a weight ratio of from 10/1to 1/10. The protective colloids mentioned can be obtained by means ofmethods known to those skilled in the art.

The protective colloid-stabilized polymer dispersions are prepared bythe emulsion polymerization process, with the polymerization temperaturegenerally being from 40° C. to 100° C., preferably from 60° C. to 90° C.In the copolymerization of gaseous comonomers such as ethylene or vinylchloride, the polymerization can also be carried out undersuperatmospheric pressure, generally in the range from 5 bar to 100 bar.The polymerization is initiated using the initiators or redox initiatorcombinations customary for emulsion polymerization, for examplehydroperoxides such as tert-butyl hydroperoxide, azo compounds such asazobisisobutyronitrile, inorganic initiators such as the sodium,potassium and ammonium salts of peroxodisulfuric acid. The initiatorsmentioned are generally used in an amount of from 0.05 to 3% by weight,based on the total weight of monomers. As redox initiators, use is madeof combinations of the abovementioned initiators with reducing agentssuch as sodium sulfite, sodium hydroxymethanesulfinate or ascorbic acid.The amount of reducing agent is preferably from 0.01 to 5.0% by weight,based on the total weight of monomers.

The polymerization mixture is stabilized by means of the abovementionedprotective colloids without additional emulsifiers. Preferably, some ofthe protective colloid is included in the initial charge and some of itis metered in after initiation of the polymerization. In general, thepolymerization is carried out in the presence of from 1 to 25% by weightof protective colloid, based on the total weight of monomers. It ispossible for all of the monomers to be charged initially, for all ofthem to be metered in or for part of them to be initially charged andthe remainder metered in after initiation of the polymerization. Asuitable method of preparing the polymer dispersions is described, forexample, in the PCT application PCT/EP98/06102, whose disclosure in thisrespect is incorporated by reference into the present application.

After conclusion of the polymerization, an after-polymerization can becarried out using known methods to remove residual monomers, for exampleby means of after-polymerization initiated using a redox catalyst.Volatile residual monomers can also be removed by means of distillation,preferably under reduced pressure, and, if desired, by passing inertentraining gases such as air, nitrogen or steam through or over thepolymerization product. The aqueous dispersions obtainable in this waygenerally contain from 1 to 25% by weight of protective colloid, basedon the polymer, and have a solids content of from 30 to 75% by weight,preferably from 40 to 65% by weight.

To prepare polymer powders which are redispersible in water, the aqueousdispersions are dried, for example by means of fluidized-bed drying,freeze drying or spray drying. The dispersions are preferably spraydried. Spray drying is carried out in customary spray drying units, andatomization can be carried out by means of single-fluid, two-fluid ormultifluid nozzles or by means of a rotating disk. The outlettemperature is generally chosen in the range from 55° C. to 100° C.,preferably from 70° C. to 90° C., depending on the unit, the T_(g) ofthe resin and the desired degree of drying.

The total amount of protective colloid prior to the drying procedure ispreferably at least 10% by weight, based on the polymer. To ensureredispersibility, it is generally necessary to add further protectivecolloids as atomization aid to the dispersion prior to drying. Ingeneral, the proportion of protective colloid prior to atomization ofthe dispersion is from 5 to 25% by weight, based on the polymer.

Suitable atomization aids are partially saponified polyvinyl acetates;polyvinylpyrrolidones; polysaccharides in water-soluble form, e.g.starches (amylose and amylopectin), modified starches such as starchethers, for example hydroxyalkyl ether starches; celluloses and theircarboxymethyl, methyl, hydroxyethyl, hydroxypropyl derivatives; proteinssuch as casein or caseinate, soya protein, gelatin; ligninosulfonates;synthetic polymers such as poly(meth)acrylic acid, copolymers of(meth)acrylates with carboxyl-functional comonomer units,poly(meth)acrylamide, polyvinylsulfonic acids and their water-solublecopolymers; melamine formaldehyde sulfonates, naphthalene formaldehydesulfonates, styrene-maleic acid and vinyl ether-maleic acid copolymers.Preferred atomization aids are partially saponified polyvinyl acetateshaving a degree of hydrolysis of from 80 to 95 mol % and a Höpplerviscosity of from 1 to 30 mPas, which may, if desired, have beenhydrophobically modified as indicated above.

In the atomization, a content of up to 1.5% by weight of antifoam, basedon the base polymer, has frequently been found to be useful. To prolongthe shelf life by improving the caking stability, in particular in thecase of powders having a low glass transition temperature, the powderobtained can be admixed with an anticaking agent, preferably in anamount of up to 30% by weight, based on the total weight of polymericconstituents. Examples of anticaking agents are calcium carbonate ormagnesium carbonate, talc, gypsum, silica and silicates, preferablyhaving particle sizes in the range from 10 nm to 10 μm.

To improve the use properties, further additives can be added in theatomization. Further constituents of dispersible powder compositionspresent in preferred embodiments are, for example, pigments, fillers,foam stabilizers, hydrophobicizing agents.

Among the types of gypsum plaster, preference is given to α- andβ-hemihydrate (CaSO₄ 1/2 H₂O) in the form of, for example, builder'splaster, stucco plaster, hard plaster of Paris or modeling plaster.However, it is also possible to modify other types of calcium sulfate,for example flooring plaster, Keene's cement, dihydrate and anhydrite.The calcium sulfate obtained in flue gas desulfurization (FGD gypsum) isalso well suited.

The gypsum plaster composition may further comprise the customaryadditives and modifiers. Customary additives for gypsum mortar arecalcium hydroxide in an amount of preferably from 1 to 30% by weight andalso inert fillers such as calcium carbonate, dolomite, clay mineralssuch as talc, mica, kaolin and/or quartz sand in customary amounts,preferably in amounts of from 5 to 90% by weight. The percentages byweight are in each case based on the total weight of the pulverulentgypsum plaster composition.

Modifiers which improve the processability of the calcium sulfate orcalcium carbonate compositions or the properties of productsmanufactured therewith are, for example, salts of long-chain fatty acidssuch as calcium stearate, sodium oleate, silicone building protectioncompositions, fungicides, fibrous materials, accelerators such asdipotassium hydrogen sulfate, retarders such as tartrates, phosphates,protein derivatives, thickeners such as cellulose ethers, starch ethers,dextrins, bentonites.

The protective colloid-stabilized vinylaromatic-1,3-diene copolymer isgenerally used in an amount of from 0.2 to 15% by weight, based on thedry weight of the formulation. To modify the gypsum-based or CaCO₃-basedmaterials, the dispersible powder or the dispersion is mixed in suitablemixers with the calcium sulfate or the calcium carbonate and possiblyfurther additives and modifiers and the mixture is homogenized.Preference is given to preparing a dry composition by means ofdispersible powders and adding the water necessary for processing at thebuilding site prior to processing.

The compositions modified according to the invention are particularlysuitable for use as gypsum mortars for knifing fillers, joint fillers,CaSO₄ flow screeds, jointing compositions, adhesive mortars or for usefor producing plasterboards or plaster molds. Examples of furtherapplications are plasters and renders or stucco work, including exteriorapplications. The usual applications for the correspondingly modifiedCaCO₃-based materials are joint fillers, gypsum-free knifing fillers andplasters and renders.

It has been found that hydrophobicization according to the inventionsignificantly improves the mechanical properties of gypsum-based orCaCO₃-based materials, e.g. adhesive pull strength, flexural strength,compressive strength, abrasion resistance, water absorption, and thusthe weathering resistance of knifing fillers.

The following examples serve to illustrate the invention:

Preparation of the Mortar Mixture:

In Examples 1 to 6, gypsum mortars or CaCO₃ mortars were prepared usingthe basic formulations 1 and 2 indicated in Tables 1 and 2. For thispurpose, the constituents of the formulation were premixed dry, theappropriate amount of water was placed in a mortar mixer and the dry mixwas stirred in.

TABLE 1 Basic formulation 1 (gypsum mortar) 660 parts by weight ofgypsum plaster (Primoplast) 300 ″ CaCO₃ (Carborex 20) 2 ″ celluloseether (CulminalC8031) 1 ″ starch ether (Amylotex7086) 1 ″ potassiumdihydrogen phosphate 450 ″ water

TABLE 2 Basic formulation 2 (CaCO₃ joint filler) 965 ″ CaCO₃ (Durcal 40)5 ″ cellulose ether (Walocel MKX) 30 ″ styrene-1,3-butadiene copolymer400 ″ water

COMPARATIVE EXAMPLE 1

60 parts by weight of an emulsifier-stabilized aqueous dispersion(solids content: 50%) of a styrene-1,3-butadiene copolymer having astyrene content of 65% by weight and a 1,3-butadiene content of 35% byweight were additionally mixed into the basic formulation 1, and 420parts by weight instead of 450 parts by weight of water were used.

COMPARATIVE EXAMPLE 2

60 parts by weight of an emulsifier-stabilized aqueous dispersion(solids content: 50%) of a styrene-acrylate copolymer (45% by weight ofstyrene; 55% by weight of butyl acrylate) were additionally mixed intothe basic formulation 1, and 420 parts by weight instead of 450 parts byweight of water were used. EXAMPLE 3

30 parts by weight of a dispersible powder based on astyrene-1,3-butadiene copolymer having a styrene content of 65% byweight and a 1,3-butadiene content of 35% by weight, which had beenstabilized using 12% by weight of a partially hydrolyzed polyvinylalcohol having a degree of hydrolysis of 88 mol % and a Höpplerviscosity of 4 mPas, were additionally mixed into the basic formulation1.

COMPARATIVE EXAMPLE 4

60 parts by weight of an emulsifier-stabilized aqueous dispersion(solids content: 50%) of a styrene-1,3-butadiene copolymer having astyrene content of 65% by weight and a 1,3-butadiene content of 35% byweight were additionally mixed into the basic formulation 2, and 370parts by weight instead of 400 parts by weight of water were used.

COMPARATIVE EXAMPLE 5

60 parts by weight of an emulsifier-stabilized aqueous dispersion(solids content: 50%) of a styrene-acrylate copolymer (45% by weight ofstyrene; 55% by weight of butyl acrylate) were additionally mixed intothe basic formulation 2, and 370 parts by weight instead of 400 parts byweight of water were used.

EXAMPLE 6

30 parts by weight of a dispersible powder based on astyrene-1,3-butadiene copolymer having a styrene content of 65% byweight and a 1,3-butadiene content of 35% by weight, which had beenstabilized using 12% by weight of a partially hydrolyzed polyvinylalcohol having a degree of hydrolysis of 88 mol % and a Höpplerviscosity of 4 mPas, were additionally mixed into the basic formulation2.

Use Tests:

Testing of the Adhesive Pull Strength:

To test the adhesive pull strength, the gypsum-based compositions orCaCO₃-based compositions from the examples were applied in a thicknessof 5 mm to concrete slabs using templates. The concrete slabs with theapplied mortars were stored under standard conditions of temperature andhumidity for 10 days. One day before the date for testing, 6 testspecimens were drilled from each slab using a drilling machine with anannular bit, diameter: 55 mm. Pull-off brackets were then adhesivelybonded onto the test specimens using a 2-component adhesive. Theadhesive pull strength was determined in accordance with DIN 18156 usinga pull-off instrument from Herion and a load increase rate of 250 N/s.The mean values for the adhesive pull strength of the individual trialsare shown in Tables 3 and 4.

Testing of the Processing Behavior:

The processing behavior on application of the gypsum- or CaCO₃-basedcompositions to the concrete slabs was evaluated subjectively usinggrades from 1 to 5, where “1” means “very good” and “5” means “poor”.The results are shown in Tables 3 and 4.

Testing of the Hardness of the Gypsum Mortars:

The hardness of the gypsum mortars was evaluated subjectively usinggrades from 1 to 5 after expiry of the abovementioned 10 day storagetime, with “1” meaning “very hard” and “5” meaning “very soft”. Theresults are shown in Tables 3 and 4.

Testing of the Water Absorption (Water Droplet Test):

To test the water absorption (hydrophobicization), a drop of waterhaving a volume of 0.5 ml was applied by means of a pipette to thesurface of the gypsum mortar after expiry of the 10 day storage time andthe time until the droplet had disappeared was determined. The resultsare shown in Tables 3 and 4.

TABLE 3 Results of tests using basic formulation 1 (gypsum mortar) C.Ex. 1 C. Ex. 2 Ex. 3 Formulation [p. by wt.] [p. by wt.] [p. by wt.]Primoplast 660 660 660 Carborex 20 300 300 300 Culminal C 8031 2 2 2Amylotex 7086 1 1 1 K(HPO₄)₂ 1 1 1 Water 420 420 450 Disp.powder/dispersion 60 60 30 Processing 4 3 2 Hardness (scratch test) 3 21 Adhesive pull strength 0.68 0.95 1.31 [N/mm²] Water droplet test [min]40 45 90

TABLE 4 Results of tests using basic formulation 2 (CaCO₃ joint filler)C. Ex. 4 C. Ex. 5 Ex. 6 Formulation [p. by wt.] [p. by wt.] [p. by wt.]Durcal 40 965 965 965 Walocel MKX 5 5 5 Water 370 370 400 Disp.powder/dispersion 60 60 30 Processing 4 3 2 Hardness (scratch test) 2 21 Adhesive pull strength 0.42 0.53 1.04 [N/mm²] Water droplet test [min]45 52 110

The results in Tables 3 and 4 show that the protectivecolloid-stabilized styrene-butadiene copolymers are superior to thosestabilized with emulsifier both in respect of processing and mechanicalhardness and in respect of hydrophobicization, both when used ingypsum-based materials and when used in CaCO₃-based materials. The sameapplies when they are compared to conventional styrene-acrylatecopolymers.

What is claimed is:
 1. A method for the modification of gypsum-basedmaterials or materials based on calcium carbonate to improve waterresistance and mechanical strength which comprises admixing with saidmaterials, protective colloid-stabilized vinylaromatic-1,3-dienecopolymers in the form of their aqueous polymer dispersions or polymerpowders which are redispersible in water, wherein the polymerdispersions or polymer powders are obtained by emulsion polymerizationof a mixture comprising at least one vinylaromatic and at least one1,3-diene, in the presence of at least one protective colloid and in theabsence of emulsifiers, and, optionally drying the aqueous polymerdispersions prior to mixing with said materials,
 2. The modifiedmaterials obtained by the method of claim
 1. 3. The modified material ofclaim 2 which is a gypsum-based material.
 4. The modified material ofclaim 2 which is based on calcium carbonate.
 5. The method as claimed inclaim 1, wherein from 20 to 80% by eight of styrene as vinylaromatic andfrom 20 to 80% by weight of 1,3-butadiene as 1,3-diene are polymerizedin the presence or absence of further comonomers.
 6. The method asclaimed in claim 1, wherein protective colloids used are one or morepolyvinyl alcohols, starches, modified starches such as starch ethers,dextrins and cyclodextrins, celluloses and their carboxymethyl, methyl,hydroxyethyl or hydroxypropyl derivatives, poly(meth)acrylic acid,poly(meth)acrylamide, melamine formaldehyde sulfonates and naphthaleneformaldehyde sulfonates.
 7. The method as claimed in claim 1, whereinthe protective colloids used are one or more unmodified orhydrophobically modified polyvinyl alcohols having a degree ofhydrolysis of from 80 to 95 mol % and a Höppler viscosity in 4% strengthaqueous solution of from 1 to 30 mpas.
 8. The method as claimed in anyof claims 1 and 5 to 7, wherein the protective colloid-stabilizedvinylaromatic-1,3-diene copolymers in the form of their aqueous polymerdispersions or polymer powders which are redispersible in water are usedin gypsum mortars for knifing fillers, joint fillers, CaSo₄ flowscreeds, jointing compositions and adhesive mortars.
 9. The method asclaimed in any of claims 1 and 5 to 7, wherein the protectivecolloid-stabilized vinylaromatic-1,3-diene copolymers in the form oftheir aqueous polymer dispersions or polymer powders which areredispersible in water are used for producing plasterboards or plastermolds.
 10. The method as claimed in any of claims 1 and 5 to 7, whereinthe protective colloid-stabilized viylaromatic-1,3-diene copolymers inthe form of their aqueous polymer dispersions or polymer powders whichare redispersible in water are used in plasters or renders or stuccowork.
 11. The method as claims in any of claims 1 and 5 to 7, whereinthe protective colloid-stabilized vinylaromatic-1,3-diene copolymers inthe form of their aqueous polymer dispersions or polymer powders whichare redispersible in water are used in joint fillers, gypsum-freeknifing fillers and plasters and renders.
 12. A process for filling ajoint in a wall surface or coating a wall surface of a buildingstructure with a settable gypsum- or calcium carbonate-based compositionexhibiting improved resistance to damage by water following setting ofthe settable composition, said process comprising providing a settablecomposition comprising gypsum, calcium carbonate, or a mixture thereof,adding to said settable composition from 0.2 to about 15 percent byweight of a solid polymer additive, said percent by weight based on thetotal dry weight of the combination of polymer additive and settablecomposition, to form a polymer-modified settable composition, said solidpolymer additive in the form of an aqueous polymer dispersion or aredispersible polymer powder, said polymer of said polymer additivecomprising a polymer of styrene and butadiene prepared in the presenceof a colloidal stabilizer, and free of emulsifiers; applying saidpolymer modified settable composition to a joint or to a wall surface;and allowing said polymer modified settable composition to set atambient temperature.
 13. The process of claim 12 wherein said wallsurface is an exterior surface of said building structure.
 14. Theprocess of claim 12, wherein said wall structure is an interior surfaceof said building structure which is exposed to dampness.
 15. The processof claim 12, wherein said colloidal stabilizer comprises a polyvinylalcohol having a degree of hydrolysis of from 85 to 94 mol percent and aHöppler viscosity in 4 weight percent aqueous solution of from 2 to 25mPa·s measured in accordance with DIN
 53015. 16. The process of claim12, wherein said colloidal stabilizer comprises a partially saponifiedpolyvinyl alcohol polymer comprising vinyl alcohol-derived units andunits derived from vinyl esters of α-branched carboxylic acids having 5carbon atoms or from 9 to 11 carbon atoms.
 17. The process of claim 12,wherein said colloidal stabilizer comprises a hydrophobicized polyvinylalcohol prepared by acetalization of a polyvinylalcohol by one or moreC₁₋₄ aldehydes.
 18. The process of claim 12, wherein said polymermodified settable composition is a plaster or stucco, optionally furthercomprising builder's lime.
 19. The process of claim 12, wherein saidpolymer modified settable composition is a joint mortar, optionallyfurther containing builder's lime.